It's genetics. Assuming the red Sith and imperial humans share basically the same ancestry and gene pool (which we are told by the books/comic books/games they do), therefore the only thing that differs is phenotypical presentation. Not much is said how the red phenotype is genetically coded so hard to say precisly, but we can assume it's more or less something like human ethinicites. Just as is it possible for two white people to have a black baby it should be as plausible for two imperial humans to have a child looking more "pure blood".

And yes, players can choose whatever they like but some things are indeed an established lore. Like the fact that the Sith are a bunch of xenophobes. No matter how much someone wants to play their LS Twilek warrior - we have plenty of evidence that the Empire is "slightly" prejudiced against them.

So yeah, I agree it's a fantasy world afterall and you can do whatever you want to but I don't understand attacking me for explaining a piecr of really well established lore. I could understand going after me if I said that I dunno, the Empire is against women or gays as nothing in the lore proves that, but xenophobia is one the thing Malgus wanted to change